Modern high speed engines, or motors, are susceptible to vibration excitation due to their high rotor rotation speeds and high operating temperatures. This has been a particular concern in aircraft gas turbine engines. In these engines, the rotors are often supported from stationary engine housings or frames by spring mountings to control rotor vibration. Each spring mounting commonly includes an anti-friction bearing and a bearing support. The radial spring rate and damping characteristics of the mountings are important factors for proper high speed engine operation of the bearing and shaft system.
In gas turbine engines, rotor shafts often encounter several resonant vibratory peaks due to rotor speed, deflected rotor shapes and minor unbalances. As a result, some radial movement of the shaft must be provided for obtaining acceptable engine vibration limits. Such movement, however, must be controlled and cushioned to prevent internal engine damage caused by contact of adjacent stationary and rotating parts. Bearing supports must, therefore, permit shafts to make small radial vibratory excursions, but prevent extreme shaft movements. Typically, a preferred cushioning spring rate for such a bearing support in a gas turbine engine is on the order of 30,000-150,000 lbs-force per inch of displacement.
One type of conventional resilient rotor bearing support comprises an axially extending cantilevered spoked cylinder or cone to support and cushion the shaft bearing. This support, commonly called a "squirrel cage" bearing support, has been used successfully to support the rotor shaft. Further, such bearing supports are designed to move undesirable vibratory resonant frequencies to rotor speeds above or below normal engine operating speeds. This reduces to a minimum the amount of time that these engines are exposed to significant vibratory resonances.
It is also useful in these types of supports to include damping in the spring mounting. Typically, the damping effect is created by supplying oil into a cavity between the bearing support and engine frame. Sometimes this involves use of an additional damping assembly having an oil filled chamber, and damping shims between the bearing support and the engine frame. An example of such a damping assembly is found in U.S. Pat. No. 4,289,360-L. I. Zirin, entitled "Bearing Damper System."
While damped resilient bearing support systems have been used successfully to control the vibratory dynamics of modern gas turbine engines, they are not always acceptable for use in certain engine configurations. For example, conventional squirrel cage bearing supports require a substantial amount of axial space for their long cantilevered spokes. This axial space is not always readily available and the placement of the squirrel cage bearing support can interfere with preferred engine gas flow paths or increase engine length and weight. Further, since the bearing is held at the end of the cantilevered axial spokes of the squirrel cage support, misalignment of the bearing support relative to the engine frame may cause uneven bearing loading and a decrease in the useful life of the bearing.
Finally, it should be noted that squirrel-cage type supports are fairly costly due to the large number of high precision machining operations needed to make the supports and associated frame features.
Another type of conventional bearing support, which is relatively compact and inexpensive, comprises a flat single layer, circular spring support, with circumferentially alternating inner and outer lobes (stand off pads) commonly called a "ring spring." Although ring springs are cheaper than squirrel cage bearing supports, they are subject to erratic performance. This is because their spring rates vary with their radial displacements, i.e., they are nonlinear devices. Further, a degree of sliding contact is inherently present between their ring lobes and the surrounding engine frame. This sliding contact can cause undesirable lobe wear that modifies the spring rate of the support. This, may in turn, result in change of rotor resonant frequency which can lead to vibratory problems. Yet further, the ring lobes provide point loadings around the circumference of the bearing which can induce undesirable stress and deflections therein. Finally, since these devices are basically flat, relatively thin circular strips, they are incapable of providing axial support for the bearings and engine rotors, and, therefore, are not effective to accommodate axial thrust loads.
Accordingly, a need exists for an inexpensive and compact bearing support which is radially resilient, which can provide axial thrust load bearing support and which is capable of controlling and damping radial vibratory motion without sliding wear points.